Roof Truss Spacing

Q.We see ourselves as quality
builders. We frame everything 16 inches on-center,
including roof trusses. Most visitors to our job sites
feel that this is overkill and that we're wasting the
customer's money. Is putting trusses 16 inches
on-center with 5/8-inch sheathing and 1/2-inch drywall
on the ceiling a thing of the past?

A.Frank Woeste, P.E.,
responds: I am not aware of any performance
issues regarding 1/2-inch drywall installed on
trusses at 16-inch versus 24-inch centers. If
interior moisture were improperly managed
— for example, if a clothes dryer were
vented into a finished garage area instead of to
the outside, a 16-inch on-center truss spacing
would be more forgiving of the bad practice.

But in general, it doesn't matter if the truss
spacing is at 12, 16, or 24 inches. What dictates
the truss design is the roof design snow load
— typically 20 or 30 psf in most of the
U.S. Once the builder or architect specifies the
loads (for example, 20-10-0-10, for top chord live,
top chord dead, bottom chord live, and bottom chord
dead loads), the truss spacing, and the desired
shape of the truss, then truss engineering takes
over.

The engineering design — usually
computer generated — dictates the size and
grade of lumber required in the chords and webs.
There are ten grades of 2x4 southern pine, for
example. If the builder requests a 16-inch
on-center spacing, a lower grade will be used that
meets the structural requirements. Conversely,
builders who request the 24-inch on-center design
should get a higher grade of lumber relative to the
16-inch on-center design. Higher grades of lumber
obviously cost more than lower grades, so it
becomes a trade-off between spacing and grade. In
general, the wider 24-inch spacing is the most
economical for residential construction.

Frank Woeste, P.E., is professor emeritus at
Virginia Tech in Blacksburg, and a frequent
contributor to JLC.